Nonaqueous electrolyte secondary battery

ABSTRACT

Provided is a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a nonaqueous electrolytic solution. The positive electrode includes a positive electrode current collector and a positive electrode active material layer that is formed on the positive electrode current collector. The positive electrode active material layer contains a positive electrode active material, an inorganic phosphate compound having ion conductivity, and a conductive material. The volatile component content in the conductive material is at least 0.15 mass % when measured according to JIS K 6221 (1982).

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2015-124944 filed onJun. 22, 2015 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonaqueous electrolyte secondarybattery.

2. Description of Related Art

In a nonaqueous electrolyte secondary battery, further improvement inenergy density has been studied in an effort to improve performance. Theimprovement in energy density can be realized, for example, byincreasing the action potential of a positive electrode to be higherthan that in the related art. However, in a case where the actionpotential of a positive electrode is set to be higher than that of ageneral nonaqueous electrolyte secondary battery, for example, to be 4.3V or higher vs. lithium metal, the durability of a battery maydeteriorate significantly. Therefore, as a result of thoroughinvestigation, the present inventors conceived a nonaqueous electrolytesecondary battery that includes an inorganic phosphate compound (aphosphate and/or a pyrophosphate) in a positive electrode (refer toJapanese Patent Application Publication No. 2014-103098 (JP 2014-103098A)). According to the above-described configuration, the elution of atransition metal element from a positive electrode active material canbe prevented, and the durability of a battery can be improved.

The present inventors repeated additional evaluation and investigationon the above-described technique. As a result, it was found that thereis room for further improvement in a case where the above-describedtechnique is applied to a battery which is used for an aspect wherehigh-rate charging and discharging is repeated. That is, in general, theelectron conductivity of an inorganic phosphate compound is extremelylow. Therefore, when a positive electrode contains an inorganicphosphate compound, the durability is improved; on the contrary, theresistance may increase. As a result, input and output characteristicsmay deteriorate.

SUMMARY OF THE INVENTION

The invention provides a nonaqueous electrolyte secondary battery havingsatisfactory input and output characteristics and high durability at thesame time.

According to the invention, there is provided a nonaqueous electrolytesecondary battery including a positive electrode, a negative electrode,and a nonaqueous electrolytic solution. The positive electrode includesa positive electrode current collector and a positive electrode activematerial layer that is formed on the positive electrode currentcollector. The positive electrode active material layer contains apositive electrode active material, an inorganic phosphate compoundhaving ion conductivity, and a conductive material. A volatile componentcontent in the conductive material is at least 0.15 mass %.

As described above, the electron conductivity of the inorganic phosphatecompound is relatively low. In general, the volatile component containedin the conductive material is likely to cause an electrochemical sidereaction to occur. Therefore, gas may be produced, and the capacitydeterioration of a battery may increase. However, according to theinvestigation of the present inventors, by allowing an inorganicphosphate compound and a conductive material having a high volatilecomponent content to be present together in a positive electrode, theabove-described concerns can be resolved, and positive effects can beobtained. That is, the electron conductivity of the positive electrodecan be improved, and the capacity deterioration can be suppressed.Accordingly, according to the above-described configuration, a batteryhaving satisfactory input and output characteristics and high durabilityat the same time can be realized.

“Volatile component content” described in this specification is a valuemeasured according to JIS K 6221 (1982). Specifically, first, ameasurement sample (conductive material) is heated and dried at 105° C.to 110° C. for 2 hours. Next, the dried measurement sample is heated andfired in a vacuum of 3 torr at 950° C. for 10 minutes. A volatilecomponent content refers to a value which is calculated from adifference (decrease in amount) between the mass values before and afterheating and firing according to the following expression.Volatile Component Content (%)=[(Mass before Heating and Firing)−(Massafter Heating and Firing)]/(Mass before Heating and Firing)×100

The volatile component is, for example, a hydroxyl group or a carboxylgroup.

In an aspect of the nonaqueous electrolyte secondary battery disclosedherein, the volatile component content in the conductive material may be1.02 mass % or lower. By suppressing the volatile component to be low,the durability can be maintained and improved. As a result, the effectsof the invention can be exhibited at a higher level.

In an aspect of the nonaqueous electrolyte secondary battery disclosedherein, in a case where a mass ratio of the volatile component contentto the total solid content of the positive electrode active materiallayer is represented by Cv (mass %), and in a case where a mass ratio ofa content of the inorganic phosphate compound to the total solid contentof the positive electrode active material layer is represented by Cp(mass %), a ratio of Cp to Cv (Cp/Cv) may be 36 to 357. By controllingthe ratio of the content of the inorganic phosphate to the volatilecomponent content in the conductive material to be in theabove-described range, the suppressing of the resistance and themaintenance and improvement of the durability can be realized at ahigher level. Accordingly, the effects of the invention can be exhibitedat a higher level.

In an aspect of the nonaqueous electrolyte secondary battery disclosedherein, a content ratio of the inorganic phosphate compound to 100 partsby mass of the positive electrode active material may be 1 part by massto 10 parts by mass. By suppressing the content ratio of the inorganicphosphate compound to be as low as possible, the resistance of thepositive electrode can be further reduced. Accordingly, moresatisfactory input and output characteristics can be realized whilemaintaining high durability.

The volatile component may be a functional group component provided on asurface of the conductive material, and the functional group componentmay be a hydrophilic functional group or a polar group. It is consideredthat the high-volatility conductive material has high affinity to asolvent, a binder, and a positive electrode active material. Therefore,an effect of improving a dispersed state of the conductive material inthe positive electrode active material layer or an effect of improvingthe adhesion between the conductive material and the positive electrodeactive material is obtained.

In an aspect of the nonaqueous electrolyte secondary battery disclosedherein, the volatile component content in the conductive material may beat least 0.24 mass % when measured according to JIS K 6221 (1982).

In an aspect of the nonaqueous electrolyte secondary battery disclosedherein, the volatile component content in the conductive material may beat least 0.35 mass % when measured according to JIS K 6221 (1982).

In an aspect of the nonaqueous electrolyte secondary battery disclosedherein, the volatile component may contain acetylene black.

As described above, the nonaqueous electrolyte secondary batterydisclosed herein (for example, a lithium ion secondary battery) hassatisfactory input and output characteristics and high durability at thesame time. Accordingly, due to the above characteristics, the nonaqueouselectrolyte secondary battery can be suitably used as, for example, apower source (power supply) for driving a vehicle-mounted motor.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a longitudinal sectional view schematically showing anonaqueous electrolyte secondary battery according to an embodiment ofthe invention;

FIG. 2 is a graph showing a relationship between a volatile componentcontent in a conductive material and an initial resistance;

FIG. 3 is a graph showing a relationship between a volatile componentcontent in a conductive material and a capacity retention; and

FIG. 4 is a graph showing a relationship between Cp/Cv and batterycharacteristics (initial resistance and capacity retention).

DETAILED DESCRIPTION OF EMBODIMENTS

A preferred embodiment of the invention will be described below. Matters(for example, a configuration of a negative electrode or a nonaqueouselectrolytic solution or general techniques relating to the constructionof a battery) necessary to practice this invention other than those (forexample, a configuration of a positive electrode) specifically referredto in this description may be understood as design matters based on therelated art in the pertinent field for a person of ordinary skill in theart. The invention can be practiced based on the contents disclosed inthis specification and common technical knowledge in the pertinentfield.

A nonaqueous electrolyte secondary battery disclosed herein includes apositive electrode, a negative electrode, and a nonaqueous electrolyticsolution. Hereinafter, the respective components will be sequentiallydescribed.

<<Positive Electrode>>

The positive electrode includes: a positive electrode current collector;and a positive electrode active material layer that is formed on thepositive electrode current collector. As the positive electrode currentcollector, a conductive member formed of highly conductive metal (forexample, aluminum or nickel) is preferably used. The positive electrodeactive material layer contains a positive electrode active material (a),an inorganic phosphate compound (b) having ion conductivity, and aconductive material (c) having at least 0.15 mass % of a volatilecomponent (herein after, also referred to as “high-volatility conductivematerial”).

By the positive electrode active material layer containing (b) theinorganic phosphate compound, at least one of the following effects (1)to (3) is exhibited: (1) a protective film containing a componentderived from the inorganic phosphate compound (for example, a filmcontaining LiF) is formed on a surface of the positive electrode activematerial through initial charging and discharging; (2) the oxidativedecomposition of a nonaqueous electrolytic solution is suppressed; and(3) an acid (for example, hydrofluoric acid) produced by the oxidativedecomposition of a nonaqueous electrolytic solution (for example, asupporting electrolyte) is trapped by the inorganic phosphate compoundto alleviate the acidity of the nonaqueous electrolytic solution. As aresult, the elution of a constituent metal element (in particular, atransition metal element) from the positive electrode active material isreduced, and the deterioration of the positive electrode active materialis suppressed.

By the positive electrode active material layer containing thehigh-volatility conductive material (c), a problem caused by theaddition of the inorganic phosphate compound (b) is compensated for, andthe conductivity of the positive electrode can be further improved. Thepresent inventors presume the reason for this to be as follows: thedispersibility of the conductive material is improved; and theconductive material is more strongly adsorbed to the surface of thepositive electrode active material. That is, the positive electrodeactive material layer is typically formed in the following procedure.First, constituent materials of the positive electrode active materiallayer and a solvent are mixed with each other to prepare a slurry. Next,the prepared slurry is applied to the positive electrode currentcollector. By drying the slurry to remove the solvent componenttherefrom, the positive electrode active material layer is formed. Here,the volatile component contained in the conductive material is afunctional group component provided on a surface of the conductivematerial. For example, the functional group component is a hydrophilicfunctional group (or a polar group) such as a hydroxyl group or acarboxyl group. Accordingly, it is considered that the high-volatilityconductive material has high affinity to a solvent, a binder, and apositive electrode active material. Therefore, it is considered that aneffect of improving a dispersed state of the conductive material in thepositive electrode active material layer or an effect of improving theadhesion between the conductive material and the positive electrodeactive material is obtained.

Further, as described above, in a case where the high-volatilityconductive material (c) is used alone, a problem may occur in anelectrochemical side reaction of the volatile component. However, byusing the inorganic phosphate compound (b) and the high-volatilityconductive material (c) in combination, this problem can be compensatedfor. Only the advantageous effects of the inorganic phosphate compound(b) and the high-volatility conductive material (c) can be utilized.Accordingly, a battery having low resistance and high durability can berealized.

As the positive electrode active material (a), one kind or two or morekinds selected from various known materials, which can be used as apositive electrode active material of a general nonaqueous electrolytesecondary battery, can be used. Preferable examples of the positiveelectrode active material include a lithium composite oxide containinglithium and at least one transition metal element.

In a preferable aspect, the positive electrode active material containsa lithium composite oxide having an action potential of 4.3 V or higher,preferably, 4.5 V or higher vs. lithium metal. As a result, the actionpotential of the positive electrode can be set to be high, and a batteryhaving high energy density can be realized. Examples of the lithiumcomposite oxide include a lithium nickel manganese composite oxidehaving a spinel structure represented by the following formula (I).Li_(x)(Ni_(y)Mn_(2-y-z)M_(z))O_(4+α)A_(q).

The formula (I) may contain or may not contain M. When M is contained, Mmay represent an arbitrary transition metal element other than Ni and Mnor a typical metal element (for example, one element or two or moreelements selected from Ti, V, Cr, Fe, Co, Cu, Zn, Al, and W).Alternatively, M may represent a metalloid element (for example, oneelement or two or more elements selected from B, Si, and Ge) or anon-metal element. By doping M with a different element other than Li,Ni, and Mn, high structural stability can be realized. In particular, itis preferable that M contains Ti and Fe. According to the investigationof the present inventors, by the formula (I) containing Ti and Fe,thermal stability is improved. Accordingly, higher durability (forexample, high-temperature cycle characteristics) can be realized.

In the formula (I), x, y, and z are values which satisfy 0.8≤x≤1.2, 0<y,0≤z, and y+z<2 (typically, y+z≤1). α is a value which is determined soas to satisfy a charge neutral condition when −0.2≤α≤0.2. q satisfies0≤q≤1. In a preferable aspect, y satisfies 0.2≤y≤1.0 (more preferably0.4≤y≤0.6; for example, 0.45≤y≤0.55). As a result, the effects of theinvention can be realized at a higher level. In another preferableaspect, z satisfies 0≤z≤1.0 (for example, 0≤z≤0.3; preferably,0.05≤z≤0.2). As a result, the effects of the invention can be realizedat a higher level. In another preferable aspect, q satisfies 0≤q≤1. In acase where q represents a value more than 0, A represents F or Cl.

Specific examples of the lithium composite oxide represented by theformula (I) include LiNi_(0.5)Mn_(1.5)O₄,LiNi_(0.5)Mn_(1.45)Ti_(0.05)O₄, LiNi_(0.45)Fe_(0.05)Mn_(1.5)O₄,LiNi_(0.45)Fe_(0.05)Mn_(1.45)Ti_(0.05)O₄, andLiNi_(0.475)Fe_(0.025)Mn_(1.475)Ti_(0.025)O₄.

A proportion of Mn in the positive electrode active material may beabout 30 mol % or higher (for example, 50 mol % or higher) with respectto 100 mol % of the total amount of all the transition metals in thepositive electrode active material. As the potential of the positiveelectrode increases, Mn is more likely to be eluted. Therefore, forexample, in a case where the positive electrode active material containsMn in the above-described proportion, it is preferable that thetechnique disclosed herein is applied. That is, in a battery includingthe positive electrode active material which contains Mn in theabove-described proportion, the above-described effect of improvingdurability is more suitably exhibited.

The form of the positive electrode active material is not particularlylimited but, typically, is a particulate or powder form. The averageparticle size of the positive electrode active material in a particulateform is preferably about 20 μm or less (typically, 1 μm to 20 μm; forexample, about 5 μm to 15 μm). In this specification, “average particlesize” refers to a particle size (also referred to as “D₅₀” or “mediansize”) corresponding to a cumulative frequency of 50 vol % in order fromthe smallest particle size in a volume particle size distribution basedon a general laser diffraction laser scattering method.

A ratio of the mass of the positive electrode active material to thetotal mass (total solid content) of the positive electrode activematerial layer is not particularly limited but is preferably about 50mass % or higher (typically, 80 mass % or higher; for example, 95 mass %or lower). As a result, high energy density and high output density canbe simultaneously realized at a high level.

As the inorganic phosphate compound, a compound having ion conductivitycan be used without any particular limitation. Preferable examples ofthe inorganic phosphate compound include a known inorganic solidelectrolyte material which can function as an electrolyte material of anall-solid-state battery. In a preferable aspect, the inorganic phosphatecompound contains at least one selected from an alkali metal element, aGroup 2 element (an alkali earth metal element in a broad sense), and ahydrogen atom. In another preferable aspect, the inorganic phosphatecompound contains at least one selected from a phosphate and apyrophosphate (diphosphate). Specific examples include: a phosphoricacid-based ion conductor such as Li₃PO₄, LiH₂PO₄, Na₃PO₄, K₃PO₄,Mg₃(PO₄)₂, Ca₃(PO₄)₂, or LiPON (lithium phosphorus oxynitride); aNASICON type ion conductor such as Li_(1.5)Al_(0.5)Ge_(1.5)(PO₄)₃; aperovskite type ion conductor; and a thio-LISICON type ion conductor.Among these, Li₃PO₄ is preferable.

The form of the inorganic phosphate compound is not particularly limitedbut, typically, is a particulate or powder form. The average particlesize of the inorganic phosphate compound in a particulate form is equalto or less than the average particle size of the positive electrodeactive material but is preferably about 20 μm or less (typically, 1 μmto 10 μm; for example, about 1 μm to 5 μm). When the average particlesize of the inorganic phosphate compound is in the above-describedrange, the inorganic phosphate compound is likely to fill gaps betweenparticles of the positive electrode active material. As a result, aconductive path between the particles of the positive electrode activematerial can be secured and the resistance of the positive electrode canbe reduced at a high level. Further, when the positive electrode activematerial and the inorganic phosphate compound are adjacent to eachother, the above-described acid trapping effect (3) can be exhibitedmore favorably. Accordingly, the deterioration of the positive electrodeactive material can be suppressed at a higher level.

A content ratio of the inorganic phosphate compound to 100 parts by massof the positive electrode active material is not particularly limitedbut is preferably about 0.1 parts by mass to 15 parts by mass(typically, 1 part by mass to 10 parts by mass; for example, 1 part bymass to 3 parts by mass). A ratio of the mass of the inorganic phosphatecompound to the total mass (total solid content) of the positiveelectrode active material layer is not particularly limited, but ispreferably about 0.1 mass % or higher (typically, 0.5 mass % or higher;for example, 1 mass % or higher) and is preferably about 15 mass % orlower (typically, 10 mass % or lower; for example, 3 mass % or lower).By controlling the content ratio of the inorganic phosphate compound tobe the predetermined value or higher, the deterioration of the positiveelectrode active material can be accurately suppressed, and the effectof improving durability can be fully exhibited. By controlling thecontent ratio of the inorganic phosphate compound to be thepredetermined value or lower, the resistance of the positive electrodecan be reduced at a higher level, and the effect of improving input andoutput characteristics can be fully exhibited.

The high-volatility conductive material (c) contains at least 0.15 mass% of a volatile component. The volatile component content is preferably0.24 mass % or higher (for example, 0.35 mass % or higher). Typically,the high-volatility conductive material has more functional groups on asurface thereof than a general conductive material. According to theinvestigation of the present inventors, the high-volatility conductivematerial has high affinity to a binder, a positive electrode activematerial, and a solvent used during the formation of the positiveelectrode active material layer. Accordingly, the high-volatilityconductive material has an effect of causing a better conductive path tobe formed between the particles of the positive electrode activematerial as compared to a conductive material (low-volatility conductivematerial) containing lower than 0.15 mass % of a volatile component. Asa result, in the positive electrode including the high-volatilityconductive material, the resistance can be reduced. The upper limitvalue of the volatile component content is not particularly limited butis typically 1.02 mass % or lower (for example, 0.78 mass % or lower).As a result, higher durability can be realized.

Japanese Patent Application Publication No. 2014-194901 (JP 2014-194901A) discloses a carbon black for a lithium ion secondary battery in whichthe average particle size of primary particles is 20 nm or less and inwhich a volatile component content is 0.20% or lower. As describedparagraph [0014] and Examples in JP 2014-194901 A, in a case where aconductive material having a high volatile component content is usedalone (in other words, in a case where the conductive material is notused in combination with the inorganic phosphate compound), theabove-described side reaction problem may become severe. Therefore, ingeneral, a conductive material having a low volatile component contentis preferable for use in a battery.

In a preferable aspect, in a case where a mass ratio of the volatilecomponent content to the total solid content of the positive electrodeactive material layer is represented by Cv (mass %), and in a case wherea mass ratio of a content of the inorganic phosphate compound (b) to thetotal solid content of the positive electrode active material layer isrepresented by Cp (mass %), a ratio of Cp to Cv (Cp/Cv) is about 10 orhigher (preferably, 30 or higher, more preferably 36 or higher; forexample, 48 or higher) and is about 500 or lower (preferably, 400 orlower, more preferably 357 or lower; for example, 250 or lower). Withinthe above-described range, the volatile component and the inorganicphosphate compound contained in the conductive material are morewell-balanced. As a result, a problem caused by an increase in thevolatile component content is compensated for, and the effects of theinvention can be exhibited at a high level. Cv described above can becalculated from the following expression.Cv (mass %)=Mass Ratio (mass %) of Content of Conductive Material toTotal Solid Content of Positive Electrode Active Material Layer×VolatileComponent Content (mass %) in Conductive Material Measured According toJIS K 6221 (1982)/100

The volatile component content in the conductive material can beadjusted, for example, using the following methods including: a method(oxidation) of heating and firing a conductive material in an oxygenatmosphere; a method of mixing a conductive material and afunctionalizing agent (for example, a compound having anoxygen-containing group) with each other and heating and firing theobtained mixture; and a method (plasma treatment) of irradiating aconductive material with oxygen plasma.

Characteristics (for example, particle size or specific surface area) ofthe high-volatility conductive material other than the volatilecomponent content is not particularly limited. In general, as theparticle size of primary particles in the conductive material decreases,the specific surface area increases. Therefore, the contact area betweenthe positive electrode active material and the conductive materialincreases, which is advantageous for forming a conductive path. On theother hand, a conductive material having a large specific surface areais likely to be bulky. Therefore, the energy density is likely todecrease, and the reactivity with the nonaqueous electrolytic solutionis likely to increase. The reason for this is as follows. The averageparticle size of primary particles of the high-volatility conductivematerial is preferably about 1 nm to 200 nm (typically, 10 nm to 100 nm;for example, 30 nm to 50 nm). The specific surface area of thehigh-volatility conductive material is preferably about 25 m²/g to 1000m²/g (typically, 50 m²/g to 500 m²/g; for example, 50 m²/g to 200 m²/g).“The average particle size of primary particles” refers to an arithmeticmean value of particle sizes which are obtained by observing 30 or more(for example, 30 to 100) primary particles with an electron microscope(any one of a scanning electron microscope and a transmission electronmicroscope may be used). “Specific surface area of an active material”refers to a value obtained by analyzing the surface area of an activematerial using a BET method (for example, one-point BET method), thesurface area being measured using a nitrogen adsorption method.

As the high-volatility conductive material, one kind or two or morekinds selected from various known materials, which can be used as aconductive material of a general nonaqueous electrolyte secondarybattery, can be used. Preferable examples of the high-volatilityconductive material include a carbon material such as carbon black (forexample, acetylene black, Ketjen black, furnace black, channel black,lamp black, or thermal black), activated carbon, graphite, or carbonfiber. Among these, carbon black is preferable from the viewpoint ofsuitably realizing the above-described characteristics (the volatilecomponent content, and the average particle size and specific surfacearea of the primary particles). In particular, acetylene black ispreferable.

A ratio of the mass of the high-volatility conductive material to thetotal mass (total solid content) of the positive electrode activematerial layer is not particularly limited but is preferably about 20mass % or lower (typically, 15 mass % or lower; for example, 10 mass %or lower; preferably, 8 mass % or lower). By controlling the contentratio of the conductive material to be the predetermined value or lower,the side reaction during charging and discharging can be furthersuppressed. Accordingly, the deterioration of the positive electrodeactive material can be suppressed at a high level, and the effect ofimproving durability can be fully exhibited. By suppressing the contentratio of the conductive material to be low, an effect of improvingenergy density can also be obtained. The lower limit of the contentratio of the high-volatility conductive material is preferably about 0.1mass % or higher (typically, 1 mass % or higher; for example, 5 mass %or higher). By controlling the content ratio of the conductive materialto be the predetermined value or higher, the electron conductivity ofthe positive electrode active material layer can be further improved.Accordingly, the resistance of the positive electrode can be reduced ata high level, and the effect of improving input and outputcharacteristics can be fully exhibited.

The positive electrode active material layer optionally contains oneoptional component or two or more optional components other than thepositive electrode active material layer (a), the inorganic phosphatecompound (b), and the conductive material (c). Typical examples of theoptional component include a binder. Examples of the binder include:vinyl halide resins such as polyvinylidene fluoride (PVdF); andpolyalkylene oxides such as polyethylene oxide (PEO). The positiveelectrode active material layer may further contain various additives(for example, an inorganic compound that produces gas duringovercharging, a dispersant, or a thickener) within a range where theeffects of the invention do not significantly deteriorate. In a casewhere the binder is used, a ratio of the mass of the binder to the totalmass (total solid content) of the positive electrode active materiallayer is not particularly limited but is typically 0.1 mass % to 10 mass% (for example, 1 mass % to 5 mass %). As a result, the mechanicalstrength (shape retaining ability) of the positive electrode can be mademore accurate.

In a preferable aspect, the open-circuit voltage (OCV) of the positiveelectrode is 4.3 V or higher, preferably 4.5 V or higher, morepreferably 4.6 V or higher, and still more preferably 4.7 V or highervs. lithium metal. By increasing OCV of the positive electrode, apotential difference (voltage) between the positive and negativeelectrodes can be made to be large, and a battery having high energydensity can be realized. The OCV of the positive electrode may be about7.0 V or lower (typically, 6.0 V or lower; for example, 5.5 V or lower)vs. lithium metal.

The open-circuit voltage of the positive electrode active material canbe measured using, for example, the following method. First, athree-electrode cell is constructed using a positive electrode as aworking electrode (WE), lithium metal as a counter electrode (CE),lithium metal as a reference electrode (RE), and a nonaqueouselectrolytic solution. Next, the three-electrode cell is adjusted in astate of charge (SOC) range of 0% to 100% based on the theoreticalcapacity of the three-electrode cell. The adjustment of the SOC can bemade by charging a portion between WE and CE, for example, using ageneral charging-discharging device or a potentiostat. The voltagebetween WE and RE in each SOC is measured and can be considered as theopen-circuit voltage (vs. Li/Li⁺).

<<Negative Electrode>>

Typically, the negative electrode includes: a negative electrode currentcollector; and a negative electrode active material layer that is formedon the negative electrode current collector. As the negative electrodecurrent collector, a conductive member formed of highly conductive metal(for example, copper or nickel) is preferably used.

The negative electrode active material layer contains a negativeelectrode active material. As the negative electrode active material,one kind or two or more kinds selected from various known materials,which can be used as a negative electrode active material of a generalnonaqueous electrolyte secondary battery, can be used. Preferableexamples of the negative electrode active material include graphite,non-graphitizable carbon (hard carbon), graphitizable carbon (softcarbon), and a carbon material having a combination thereof (forexample, graphite having a surface coated with amorphous carbon). Amongthese, a graphite-based carbon material is preferable in which graphiteaccounts for 50 mass % or higher in the total mass of the negativeelectrode active material.

The negative electrode active material layer optionally contains oneoptional component or two or more optional components other than thenegative electrode active material. Typical examples of the optionalcomponent include a binder. Preferable examples of the binder include:rubbers such as styrene-butadiene rubber (SBR); fluororesins such aspolyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); andcellulose polymers such as carboxymethyl cellulose (CMC). The negativeelectrode active material layer optionally further contains variousadditives (for example, a dispersant, a thickener, and a conductivematerial).

<<Nonaqueous Electrolytic Solution>>

The nonaqueous electrolytic solution is liquid at normal temperature(for example, 25° C.). It is preferable that the nonaqueous electrolyticsolution is typically liquid in an operating temperature range (forexample, −20° C. to +60° C.) of the battery. In the nonaqueouselectrolytic solution, it is preferable that a nonaqueous solventcontains a supporting electrolyte. As the supporting electrolyte, onekind or two or more kinds selected from various known compounds, whichcan be used as a supporting electrolyte of a general nonaqueouselectrolyte secondary battery, can be appropriately adopted. Forexample, in a case where lithium ions (Li⁺) are used as the chargecarriers, examples of the supporting electrolyte include lithium saltssuch as LiPF₆, LiBF₄, LiClO₄, and Li(CF₃SO₂)₂N. Among these, LiPF₆ ispreferable. The concentration of the supporting electrolyte ispreferably 0.7 mol/L to 1.3 mol/L.

As the nonaqueous solvent, one kind or two or more kinds selected fromvarious organic solvents, which can be used as a nonaqueous solvent of ageneral nonaqueous electrolyte secondary battery, can be appropriatelyadopted. Specific examples of the nonaqueous solvent include carbonates,ethers, esters, nitriles, sulfones, and lactones. Among these, a solventhaving satisfactory oxidation resistance (that is, a high oxidativedecomposition potential) is preferable. Preferable examples of thesolvent include a fluorine-based solvent (fluorine-containing nonaqueoussolvent). As the fluorine-based solvent, an organic solvent having achemical structure in which at least one hydrogen atom of an organicsolvent not containing fluorine is substituted with a fluorine atom canbe considered.

Among the fluorine-based solvents, fluorinated carbonate is particularlypreferable. By the nonaqueous electrolytic solution containingfluorinated carbonate, the oxidation potential of the nonaqueouselectrolytic solution can be improved more favorably. As a result, evenin a case where the positive electrode potential increases (for example,4.3 V or higher (vs. Li/Li⁺)), the oxidative decomposition of thenonaqueous electrolytic solution can be significantly suppressed. In apreferable aspect, a ratio of the mass of the fluorine-based solvent tothe total mass of the nonaqueous solvent is about 10 mass % or higher(for example, 30 mass % to 100 mass %; preferably 50 mass % to 100 mass%). For example, the ratio may be 100 mass % (for example, 99 mass % orhigher).

Examples of the fluorinated carbonate include: fluorinated cycliccarbonates such as monofluoroethylene carbonate (MFEC) anddifluoroethylene carbonate (DFEC); and fluorinated chain carbonates suchas fluorodimethyl carbonate, difluorodimethyl carbonate,trifluorodimethyl carbonate (TFDMC), and fluoromethyl difluoromethylcarbonate. In a preferable aspect, the nonaqueous electrolytic solutioncontains at least one fluorinated chain carbonate and at least onefluorinated cyclic carbonate. The fluorinated chain carbonate(preferably, fluorinated linear carbonate) is effective for suppressingthe viscosity of the nonaqueous electrolytic solution to be low. Thefluorinated cyclic carbonate is effective for improving the ionconductivity of the nonaqueous electrolytic solution. A mixing ratio ofthe fluorinated chain carbonate to the fluorinated cyclic carbonate ispreferably about 1:3 to 3:1 (for example, about 1:1). As a result, theabove-described characteristics can be made to be well-balanced at ahigh level.

The nonaqueous electrolytic solution may appropriately contain acomponent other than the nonaqueous solvent and the supportingelectrolyte. Examples of the optional component include: a film formingagent such as lithium bis(oxalato)borate (LiBOB), vinylene carbonate(VC), vinyl ethylene carbonate (VEC), or fluoroethylene carbonate (FEC);a compound which may produce gas during overcharging, such as biphenyl(BP) or cyclohexylbenzene (CHB); and a viscosity modifier.

<<Embodiment of Nonaqueous Electrolyte Secondary Battery>>

Although not particularly limited thereto, a schematic configuration ofa nonaqueous electrolyte secondary battery according to an embodiment ofthe invention will be described in detail using a nonaqueous electrolytesecondary battery (single cell), which is schematically shown in FIG. 1,as an example. In the following drawings, parts or portions having thesame function are represented by the same reference numerals, and therepeated description will not be made or will be simplified. In eachdrawing, a dimensional relationship (for example, length, width, orthickness) does not necessarily reflect the actual dimensionalrelationship.

FIG. 1 is a longitudinal sectional view showing a nonaqueous electrolytesecondary battery (single cell) 100 according to an embodiment of theinvention. The nonaqueous electrolyte secondary battery 100 includes: aflat wound electrode body 80; a nonaqueous electrolytic solution (notshown); and a flat cuboid battery case 50 that accommodates the woundelectrode body 80 and the nonaqueous electrolytic solution. The batterycase 50 includes: a flat cuboid battery case body 52 having an openupper end; and a lid 54 that covers the opening. A material of thebattery case 50 may be a light-weight metal such as aluminum. A shape ofthe battery case is not particularly limited and may be, for example, acuboid shape or a cylindrical shape. On a top surface (that is, the lid54) of the battery case 50, a positive electrode terminal 70 and anegative electrode terminal 72 are provided for external connection. Aportion of the positive electrode terminal 70, the negative electrodeterminal 72 protrudes toward the surface side of the lid 54. Thepositive electrode terminal 70 is electrically connected to the positiveelectrode sheet 10 of the wound electrode body 80 on the battery case 50side. The negative electrode terminal 72 is electrically connected tothe negative electrode sheet 20 of the wound electrode body 80 on thebattery case 50 side. The lid 54 further includes a safety valve 55 fordischarging gas, produced from the inside of the battery case 50, to theoutside of the battery case 50.

The wound electrode body 80 includes an elongated positive electrodesheet 10 and an elongated negative electrode sheet 20. The positiveelectrode sheet 10 includes: an elongated positive electrode currentcollector; and a positive electrode active material layer 14 that isformed on a surface (typically, on both surfaces) of the positiveelectrode current collector in the longitudinal direction. The negativeelectrode sheet 20 includes: an elongated negative electrode currentcollector; and a negative electrode active material layer 24 that isformed on a surface (typically, on both surfaces) of the negativeelectrode current collector in the longitudinal direction. The woundelectrode body 80 includes two elongated separator sheets 40. Thepositive electrode sheet 10 (positive electrode active material layer14) and the negative electrode sheet 20 (negative electrode activematerial layer 24) are insulated by the separator sheets 40. A materialof the separator sheets 40 may be a resin such as polyethylene (PE),polypropylene (PP), polyester, cellulose, or polyamide. For example, inorder to prevent internal short-circuiting, a porous heat resistancelayer containing inorganic compound particles (inorganic filler) may beprovided on a surface of the separator sheet 40. This wound electrodebody 80 may be flat and, for example, can adopt an appropriate shape andan appropriate configuration according to the shape and intended use ofthe battery.

<<<Use of Nonaqueous Electrolyte Secondary Battery>>

The nonaqueous electrolyte secondary battery disclosed herein exhibitsatisfactory input and output characteristics and high durability at thesame time. The nonaqueous electrolyte secondary battery disclosed hereincan be used for various applications. Due to the above-describedcharacteristics, the nonaqueous electrolyte secondary battery can besuitably used for an application where satisfactory input and outputdensities and high durability are required at the same time. Examples ofthe application include a power source (power supply) for driving amotor mounted in a vehicle such as a plug-in hybrid vehicle, a hybridvehicle, or an electric vehicle.

Hereinafter, several examples relating to the invention will bedescribed, but the examples are not intended to limit the invention.

[I. Investigation Relating to Volatile Component of Conductive Material]

<Preparation of Positive Electrodes(Examples 1 to 5, Reference Examples1 and 2)>

As a positive electrode active material, NiMn spinel(LiNi_(0.5)Mn_(1.5)O₄, Ti or Fe-doped product) having an averageparticle size of 7 μm was prepared. As an inorganic phosphate compound,commercially available Li₃PO₄ having an average particle size of 3 μmwas prepared. As conductive materials, seven kinds of acetylene blackhaving a volatile component content of 0.01 mass % to 1.02 mass % in theconductive materials were prepared. The volatile component content inacetylene black which was generally used was about 0.01 mass %. First,NiMn spinel and Li₃PO₄ described above were mixed with each other at amass ratio of 100:3. The mixture; acetylene black (AB) having a volatilecomponent content as shown in Table 1; and polyvinylidene fluoride(PVdF) as a binder were weighed such that a mass ratio((LiNi_(0.5)Mn_(1.5)O₄+Li₃PO₄):AB:PVdF) thereof was 89:8:3. Theseweighed materials were mixed with N-methyl-2-pyrrolidone (solvent; NMP)to prepare a slurry. This slurry for forming a positive electrode activematerial layer was applied to an aluminum foil (positive electrodecurrent collector) and was dried. As a result, a positive electrodeincluding a positive electrode active material layer that is formed onthe positive electrode current collector was obtained.

<Preparation of Positive Electrode (Reference Examples 3 to 7)>

In Reference Examples 3 to 7, positive electrodes were prepared withoutusing Li₃PO₄. That is, the above-described NiMn spinel; acetylene black(AB) having a volatile component content as shown in Table 1; andpolyvinylidene fluoride (PVdF) were weighed such that a mass ratio(LiNi_(0.5)Mn_(1.5)O₄:AB:PVdF) thereof was 89:8:3. These weighedmaterials were mixed with N-methyl-2-pyrrolidone (solvent; NMP) toprepare a slurry. This slurry for forming a positive electrode activematerial layer was applied to an aluminum foil (positive electrodecurrent collector) and was dried. As a result, a positive electrodeincluding a positive electrode active material layer that is formed onthe positive electrode current collector was obtained.

<Preparation of Negative Electrode>

Graphite (C) as a negative electrode active material; carboxymethylcellulose (CMC) as a binder; styrene-butadiene rubber (SBR) were weighedsuch that a mass ratio (C:CMC:SBR) thereof was 98:1:1. The weighedmaterials were mixed with ion exchange water (solvent) to prepare aslurry. This slurry for forming a negative electrode active materiallayer was applied to a copper foil (negative electrode currentcollector) and was dried. As a result, a negative electrode including anegative electrode active material layer that is formed on the negativeelectrode current collector was obtained.

<Construction of Nonaqueous Electrolyte Secondary Battery>

The positive electrode and the negative electrode prepared as describedabove were laminated with a separator interposed therebetween to preparean electrode body. As the separator, a porous film having a three-layerstructure of polypropylene (PP)/polyethylene (PE)/polypropylene (PP) wasused. In order to prepare a nonaqueous electrolytic solution, LiPF₆ as asupporting electrolyte was dissolved in a mixed solvent such that theconcentration thereof was 1.0 mol/L, the mixed solvent containingmonofluoroethylene carbonate (MFEC) as a cyclic carbonate andtrifluorodimethyl carbonate (TFDMC) as a chain carbonate at a volumeratio of 50:50. The electrode body and the nonaqueous electrolyticsolution which were prepared as described above were sealed into alaminate battery case. In this way, lithium ion secondary batteries(Examples 1 to 5 and Reference Examples 1 to 7) were constructed.

<Conditioning Treatment>

The following charging-discharging operations (1) and (2) wererepeatedly performed on the constructed batteries in 3 cycles in atemperature environment of 25° C. to perform a conditioning treatment.(1) The batteries were charged to 4.9 V at a constant current (CC) at arate of 1/3 C, and then the operation was stopped for 10 minutes. (2)The batteries were discharged to 3.5 V at a constant current (CC) at arate of 1/3 C, and then the operation was stopped for 10 minutes.

<Initial Resistance>

The SOC of the batteries after the conditioning treatment was adjustedto 60% in a temperature environment of 25° C. Each of the batteries wasdischarged at a constant current (CC) at a rate of 1 C, 3 C, 5 C, or 10C, and a voltage change (voltage drop) thereof was measured for 10seconds from the start of the discharge. The measured voltage changeamount (V) was divided by the corresponding current value to calculatean IV resistance. Here, the arithmetic mean value of the IV resistancewas set as an initial resistance (Ω). The results are shown in Table 1.FIG. 2 is a graph showing a relationship between a volatile componentcontent in a conductive material and an initial resistance.

<High-Temperature Durability Test>

Next, after the initial resistance measurement, the batteries were leftto stand in a thermostatic chamber set to a temperature of 60° C. In atemperature environment of 60° C., the following charging anddischarging operations (1) and (2) were performed on the batteries in200 cycles. (1) The batteries were discharged to 4.75 V at a constantcurrent (CC) at a rate of 2 C, and then the operation was stopped for 10minutes. (2) The batteries were discharged to 3.5 V at a constantcurrent (CC) at a rate of 2 C, and then the operation was stopped for 10minutes. After completion of the high-temperature durability test, thecapacity retention (%) was calculated from a ratio of the CC dischargecapacity of the 200th cycle to the CC discharge capacity of the firstcycle according to the following expression.((CC Discharge Capacity of 200th Cycle/CC Discharge Capacity of FirstCycle)×100(%))

The results are shown in Table 1. FIG. 3 is a graph showing arelationship between a volatile component content in a conductivematerial and a capacity retention.

TABLE 1 Conductive Material Volatile Component Li₃PO₄ Content** inVolatile Battery Characteristics Content Content Conductive ComponentInitial Capacity (Part(s) Cp* Material Content Content Cv* ResistanceRetention by Mass) (mass %) (mass %) (mass %) (mass %) Cp/Cv (Ω) (%)Reference 3.0 2.91 0.01 8 0.001 3750 1.64 86.0 Example 1 Reference 3.02.91 0.04 8 0.003 938 1.65 86.1 Example 2 Example 1 3.0 2.91 0.15 80.012 250 1.53 86.5 Example 2 3.0 2.91 0.24 8 0.019 156 1.48 86.4Example 3 3.0 2.91 0.35 8 0.028 107 1.45 86.8 Example 4 3.0 2.91 0.78 80.062 48 1.46 86.5 Example 5 3.0 2.91 1.02 8 0.082 37 1.44 87.0Reference 0 0 0.01 8 0.001 — 1.26 73.0 Example 3 Reference 0 0 0.04 80.003 — 1.26 73.1 Example 4 Reference 0 0 0.15 8 0.012 — 1.23 72.3Example 5 Reference 0 0 0.24 8 0.019 — 1.23 71.4 Example 6 Reference 0 00.35 8 0.028 — 1.24 70.5 Example 7 *Mass ratio to the total solidcontent of the positive electrode active material layer **Volatilecomponent content in conductive material which is measured according toJIS K 6221 (1982)

Reference Examples 3 to 7 were test examples for comparing andinvestigating the effects of a volatile component content in aconductive material in a system in which the positive electrode did notcontain Li₃PO₄. As can be seen from Table 1 and FIG. 2, in the systemnot containing Li₃PO₄, the initial resistance was suppressed to be low.However, as can be seen from Table 1 and FIG. 3, in the system notcontaining Li₃PO₄, as the volatile component content in the conductivematerial increased, high-temperature durability (capacity retention)deteriorated. The reason for this is presumed that the volatilecomponent electrochemically reacted (was decomposed). It is presumedthat a by-product was produced due to the above-described reaction,which had adverse effects on battery characteristics.

Reference Examples 1 and 2 and Examples 1 to 5 were test examples forcomparing and investigating the effects of a volatile component contentin a conductive material in a system in which the positive electrodecontained Li₃PO₄. As can be seen from Table 1 and FIG. 2, in the systemcontaining Li₃PO₄, when the volatile component content in the conductivematerial is a predetermined value or higher, the initial resistance wassignificantly reduced. In particular, in a case where the volatilecomponent content in acetylene black as the conductive material is 0.15mass % or higher, preferably 0.24 mass % or higher, and more preferably0.35 mass % or higher, an effect of reducing the resistance wassignificant. It is presumed that from the above results that, in thesystem containing Li₃PO₄, the electron conductivity of the positiveelectrode has a large effect on the battery resistance. It is presumedthat, by using the high-volatility conductive material, low electronconductivity of Li₃PO₄ can be compensated for, and a good conductivepath can be formed in the positive electrode active material layer. Ascan be seen from Table 1 and FIG. 3, in the systems containing Li₃PO₄,the capacity retention exhibited a high value. That is, unlike ReferenceExamples 3 to 7, a correlation with the volatile component content wasnot established. The reasons for this are not clear, but one of thereasons is presumed to be that the by-product derived from the volatilecomponent was trapped by Li₃PO₄; as a result, capacity deterioration wassuppressed.

FIG. 4 is a graph showing a relationship between Cp/Cv and batterycharacteristics (initial resistance and capacity retention). As can beseen from Table 1 and FIG. 4, when a ratio (Cp/Cv) of the mass ratio Cpof the inorganic phosphate compound to the mass ratio Cv of the volatilecomponent content is about 500 or lower, for example, 30 to 400, thevolatile component and the inorganic phosphate compound can be made tobe more well-balanced in the conductive material. As a result, theeffect of reducing the initial resistance and the effect of improvingthe durability can be realized at the same time at a high level.

[II. Investigation Relating to Content Ratio of Inorganic PhosphateCompound]

Lithium ion secondary batteries of Examples 6 and 7 were constructedusing the same method as in Example 3, except that the mass ratio ofLi₃PO₄ was changed as shown in Table 2. Lithium ion secondary batteriesof Reference Examples 8 and 9 were constructed using the same method asin Reference Example 1, except that the mass ratio of Li₃PO₄ was changedas shown in Table 2. Using the same method as in Example 3 and ReferenceExample 3, the initial resistance measurement and the high-temperaturedurability test were performed. The results are shown in Table 2.

TABLE 2 Conductive Material Volatile Component Li₃PO₄ Content** inVolatile Battery Characteristics Content Content Conductive ComponentInitial Capacity (Part(s) Cp* Material Content Content Cv* ResistanceRetention by Mass) (mass %) (mass %) (mass %) (mass %) Cp/Cv (Ω) (%)Reference 0 0 0.35 8 0.028 — 1.24 70.5 Example 7 Example 6 1 0.99 0.35 80.028 36 1.4 82.8 Example 3 3 2.91 0.35 8 0.028 107 1.45 86.8 Example 710 9.09 0.35 8 0.028 357 1.49 85.0 Reference 0 0 0.01 8 0.001 — 1.2673.0 Example 3 Reference 1 0.99 0.01 8 0.001 1250 1.58 83.0 Example 8Reference 3 2.91 0.01 8 0.001 3750 1.64 86.0 Example 1 Reference 10 9.090.01 8 0.001 12500 1.62 85.1 Example 9 *Mass ratio to the total solidcontent of the positive electrode active material layer **Volatilecomponent content in conductive material which is measured according toJIS K 6221 (1982)

Reference Example 7 and Examples 6, 3, and 7 were test examplescontaining a conductive material having a volatile component content of0.15 mass % or higher (high-volatility conductive material). ReferenceExample 3, 8, 1, and 9 were test examples containing a conductivematerial having a volatile component content of lower than 0.15 mass %(low-volatility conductive material). As can be seen from Table 2, thecapacity retentions of Examples 6, 3, and 7 and Reference Examples 8, 1,and 9 containing Li₃PO₄ were higher than those of Reference Examples 7and 3 not containing Li₃PO₄. When the test examples containing thehigh-volatility conductive material are compared to the test examplescontaining the low-volatility conductive material, the initialresistances of the test examples containing the high-volatilityconductive material were suppressed to be relatively low. As can be seenfrom the results of Reference Example 7 and Examples 6, 3, and 7, whenthe Li₃PO₄ content is in a range of 1 part by mass to 10 parts by masswith respect to 100 parts by mass of the positive electrode activematerial, the effect of improving the durability and the effect ofreducing the initial resistance can be made to be more well-balanced.

Hereinabove, the invention has been described in detail, but theabove-described embodiment and examples are merely exemplary. Theinvention disclosed herein includes various modifications andalternations of the above-described specific examples.

What is claimed is:
 1. A nonaqueous electrolyte secondary batterycomprising: a positive electrode; a negative electrode; and a nonaqueouselectrolytic solution, wherein the positive electrode includes apositive electrode current collector and a positive electrode activematerial layer that is formed on the positive electrode currentcollector, the positive electrode active material layer contains apositive electrode active material, an inorganic phosphate compoundhaving ion conductivity, and a conductive material, and a volatilecomponent content in the conductive material is 0.15 mass % to 0.78 mass%, wherein in a case where a mass ratio of the volatile componentcontent to a total solid content of the positive electrode activematerial layer is represented by Cv, and in a case where a mass ratio ofa content of the inorganic phosphate compound to the total solid contentof the positive electrode active material layer is represented by Cp, aratio of Cp to Cv is 36 to
 357. 2. The nonaqueous electrolyte secondarybattery according to claim 1, wherein a content of the inorganicphosphate compound to 100 parts by mass of the positive electrode activematerial is 1 part by mass to 10 parts by mass.
 3. The nonaqueouselectrolyte secondary battery according to claim 1, wherein the volatilecomponent is a functional group component provided on a surface of theconductive material, and the functional group component is a hydrophilicfunctional group or a polar group.
 4. The nonaqueous electrolytesecondary battery according to claim 1, wherein the volatile componentcontent in the conductive material is at least 0.24 mass %.
 5. Thenonaqueous electrolyte secondary battery according to claim 1, whereinthe volatile component content in the conductive material is at least0.35 mass %.